Powder for capacitor, sintered body and capacitor using the sintered body

ABSTRACT

A niobium primary powder having an average particle size of 0.01 to 0.5 μm and an average circular degree of 0.8 or more, the circular degree being defined by 4 πA/L 2  (wherein A is an area of a solid projected on a plain face and L is an outer circumferential length of the projection view); a niobium primary agglomerated powder having an average particle size of 0.03 to 20 μm, which is an agglomerate of the niobium primary powder; a niobium secondary agglomerated powder having an average particle size of 50 to 150 μm, which is obtained by granulating the primary agglomerated powder; a sintered body of the niobium primary agglomerated powder or niobium secondary agglomerated powder; and a capacitor using the sintered body. By using sintered bodies of the niobium primary agglomerated powder or niobium secondary agglomerated powder, a capacitor having a large capacitance per unit volume and good voltage resistance can be manufactured.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is an application based on the provisions of 35U.S.C. Article 111(a), claiming the benefit from U.S. provisionalapplication Ser. No. 60/268,914 filed on Feb. 16, 2001, under theprovisions of 35 U.S.C. 111(b), pursuant to 35 U.S.C. Article 119(e)(1).

TECHNICAL FIELD

[0002] The present invention relates to a niobium powder and a niobiumsintered body, which can provide a capacitor having good voltageresistance characteristics and a large capacitance per unit volume, andalso relates to a capacitor using the sintered body.

BACKGROUND ART

[0003] Capacitors for use in electronic instruments such as portabletelephone and personal computer are demanded to have a small size and alarge capacitance. Among these capacitors, a tantalum capacitor ispreferred because of its large capacitance for the size and goodperformance. In this tantalum capacitor, a sintered body of tantalumpowder is generally used for the anode moiety. In order to increase thecapacitance of the tantalum capacitor, it is necessary to increase theweight of the sintered body or to use a sintered body increased in thesurface area by pulverizing the tantalum powder.

[0004] The former method of increasing the weight of the sintered bodynecessarily involves enlargement of the capacitor shape and cannotsatisfy the requirement for downsizing. On the other hand, in the lattermethod of pulverizing tantalum powder to increase the surface area, thepore size of the tantalum sintered body decreases or closed poresincrease at the stage of sintering and therefore, impregnation of thecathode agent in the later process becomes difficult.

[0005] As one of means for solving these problems, a capacitor using asintered body of powder of materials having a dielectric constant largerthan that of tantalum is being studied. Niobium is known as a materialhaving such a large dielectric constant.

[0006] The capacitor (niobium capacitor) manufactured using a niobiumpowder as a raw material is considered, however, inferior in the voltageresistance characteristics to capacitors manufactured using a tantalumpowder as a raw material. Furthermore, when the average particle size ofniobium powder as a raw material of niobium capacitors is reduced so asto increase the capacitance, a cathode agent cannot be easilyimpregnated in the later process, similarly to the case of tantalumpowder.

[0007] With respect to metals such as niobium, JP-A-6-25701 (U.S. Pat.No. 5,407,458) (the term “JP-A” as used herein means an “unexaminedpublished Japanese patent application”) discloses a fine metal powderhaving a particle size of 1.0 nm to 3 μm, however, this patentpublication is silent on the shape (circular degree) of the fine powder,the agglomerated powder of the fine powder and the sintered body forcapacitors using the powder.

DISCLOSURE OF THE INVENTION

[0008] As a result of extensive investigations to solve theabove-described problems, the present inventors have found that aniobium capacitor using a sintered body of agglomerated powder ofniobium particles (primary powder) having a specific particle sizeexhibits good voltage resistance characteristics and is increased in thecapacitance per unit volume, in particular, when the niobium particleused as a starting material of the sintered body is a primary powderhaving a high circular degree or a secondary agglomerated powderobtained by re-granulating the niobium primary powder, the effects aremore enhanced. The present invention has been accomplished based on thisfinding.

[0009] More specifically, the present invention provides a powder forcapacitors, having the following construction, a sintered body thereofand a capacitor using the sintered body.

[0010] 1. A niobium primary powder for capacitors, having an averagecircular degree of 0.8 or more and an average particle size of 0.01 to0.5 μm, the circular degree being defined by the following formula:

Circular degree=4 πA/L²

[0011] (wherein A is an area of a solid projected on a flat surface andL is the outer circumferential length of the projection view).

[0012] 2. The niobium primary powder for capacitors as described in 1above, wherein at least a part of niobium is nitrided.

[0013] 3. A niobium primary agglomerated powder for capacitors, havingan average primary particle size of 0.01 to 0.5 μm and an averageparticle size of 0.03 to 20 μm.

[0014] 4. The niobium primary agglomerated powder for capacitors asdescribed in 3 above, wherein the primary particles have an averagecircular degree of 0.8 or more, the circular degree being defined by thefollowing formula:

Circular degree=4 πA/L²

[0015] (wherein the symbols have the same meanings as in 1 above)

[0016] 5. The niobium primary agglomerated powder for capacitors asdescribed in 3 or 4 above, wherein at least a part of niobium isnitrided.

[0017] 6. The niobium primary agglomerated powder for capacitors asdescribed in 3 or 4 above, wherein the specific surface area is from 4to 30 m²/g.

[0018] 7. A niobium secondary agglomerated powder for capacitors,obtained by granulating the niobium primary agglomerated powderdescribed in any one of 3 to 6 above.

[0019] 8. The niobium secondary agglomerated powder for capacitors asdescribed in 7 above, wherein the specific surface area is from 3 to 20m²/g.

[0020] 9. The niobium secondary agglomerated powder for capacitors asdescribed in 7 or 8 above, wherein the average particle size is from 50to 150 μm.

[0021] 10. The niobium secondary agglomerated powder for capacitors asdescribed in any one of 7 to 9 above, wherein at least a part of niobiumis nitrided.

[0022] 11. A sintered body using the niobium primary agglomerated powderfor capacitors described in any one of 3 to 6 above.

[0023] 12. A sintered body using the niobium secondary agglomeratedpowder for capacitors described in any one of 7 to 10 above.

[0024] 13. The sintered body as described in 11 or 12 above, wherein thespecific surface area is from 1 to 10 m²/g.

[0025] 14. A capacitor comprising the sintered body described in 11 or12 above as one part electrode, a dielectric material formed on thesurface of the sintered body, and another part electrode provided on thedielectric material.

[0026] 15. The capacitor as described in 14 above, wherein thedielectric material mainly comprises niobium oxide.

[0027] 16. The capacitor as described in 15 above, wherein the niobiumoxide is formed by electrolytic oxidation.

[0028] 17. The capacitor as described in any one of 14 to 16 above,wherein the another part electrode is at least one material (compound)selected from the group consisting of an electrolytic solution, anorganic semiconductor and an inorganic semiconductor.

[0029] 18. The capacitor as described in 17 above, wherein the anotherpart electrode is composed of an organic semiconductor and the organicsemiconductor is at least one organic semiconductor selected from thegroup consisting of an organic semiconductor comprising a benzopyrrolinetetramer and chloranile, an organic semiconductor mainly comprisingtetrathiotetracene, an organic semiconductor mainly comprisingtetracyanoquinodimethane, and an organic semiconductor mainly comprisingan electrically conducting polymer obtained by doping a dopant into apolymer containing two or more repeating units represented by thefollowing formula (1) or (2):

[0030] wherein R¹ to R⁴, which may be the same or different, eachrepresents a hydrogen atom, an alkyl group having from 1 to 6 carbonatoms or an alkoxy group having from 1 to 6 carbon atoms, X representsan oxygen atom, a sulfur atom or a nitrogen atom, R⁵ is present onlywhen X is a nitrogen atom and represents a hydrogen atom or an alkylgroup having from 1 to 6 carbon atoms, and each of the pairs R¹ and R²,and R³ and R⁴ may combine with each other to form a ring.

[0031] 19. The capacitor as described in 18 above, wherein the organicsemiconductor is at least one member selected from polypyrrole,polythiophene polyaniline and substitution derivatives thereof.

EMBODIMENT OF THE INVENTION

[0032] The niobium powder as the starting material of the sintered bodyfor niobium capacitors according to the present invention includes threekinds of niobium powders, namely, a niobium primary powder specified inthe shape (circular degree) of primary particle, a granulated powderresulting from granulating the niobium primary powder to an appropriatesize (hereinafter simply referred to as “primary agglomerated powder”)and a granulated powder resulting from re-granulating the primaryagglomerated powder to an appropriate size (hereinafter simply referredto as “secondary agglomerated powder”).

[0033] [Nioblum Primary Powder]

[0034] The niobium primary powder of the present invention has anaverage particle size of 0.01 to 0.5 μm, preferably from 0.05 to 0.5 μm.If the average particle size is less than 0.01 μm, even with theagglomerated powder structure of the present invention, the cathodeagent cannot be easily impregnated in the later process, whereas if itexceeds 0.5 μm, a small-size and large-capacitance capacitor can behardly obtained.

[0035] When a capacitor is manufactured using a sintered body of aprimary agglomerated powder comprising this primary powder, particularlypowder particles having an average circular degree of 0.8 or more,preferably 0.84 or more, the obtained capacitor can have excellentvoltage resistance characteristics. If the average circular degree isless than the above-described range, the capacitor obtained by using theprimary agglomerated powder resulting from granulating the primarypowder may be sometimes insufficiently improved in the voltageresistance characteristics.

[0036] However, even if the primary powder has an average circulardegree of less than 0.8, when the primary agglomerated powder of thisprimary powder is further granulated to prepare a secondary agglomeratedpowder and a capacitor is manufactured using a sintered body obtained bysintering the secondary agglomerated powder, the capacitor can also haveexcellent voltage resistance characteristics.

[0037] In the present invention, the average circular degree is anaverage value of the circular degree defined by the following formulaand this is used as an index for showing the shape of powder particle ofthe primary powder.

Circular degree=4 πA/L²

[0038] (wherein A is an area (μm²) of a solid projected on a flatsurface and L is an outer circumferential length of the projectionview).

[0039] In practice, the circular degree can be determined as follows. Apicture of powder particles is taken by a scanning electron microscope(SEM) and in recognition of this SEM photograph as a projection view ofthe powder particles, the area and the circumferential length ofindividual particles are measured on the picture. In order to determinethe average circular degree with good precision, 100 or more, preferably1,000 or more particles are measured on the circular degree and from thevalues obtained, an average value is calculated. For example, when anenlarged SEM photograph of powder particles is taken at a magnificationof about 2,000 times and individual particles are determined on the areaand the circumferential length by a computer processing, even particleshaving a complicated outer circumferential shape can be counted andtherefore, the determined average circular degree is reliable. Accordingto the above-described formula, the circular degree of a completelyspherical particle is 1 and the circular degree of a cubic particle is0.78. As the average circular degree is closer to 1, the shape of thepowder particle is closer to a sphere.

[0040] Examples of the method for producing a primary powder having anaverage particle size of 0.01 to 0.5 μm and an average circular degreeof 0.8 or more include a method of pulverizing a hydrogenated niobiumpowder in a jet mill. By controlling the residence time of niobiumpowder in the jet mill, the average circular degree can be set to apredetermined preferable value. The hydrogenated niobium powder is aniobium powder obtained by allowing a hydrogen gas to be absorbed into aniobium powder (having an average particle size of 0.5 to tens of μm)produced using a known method. The niobium powder is dehydrogenatedafter the pulverization in the jet mill or after forming a primaryagglomerated powder which is described later, whereby the objectiveprimary powder can be obtained.

[0041] At least a part of the niobium primary powder is preferablynitrided. The nitrided amount is from tens of ppm by mass to tens ofthousands of ppm by mass. By this partial nitridation, the capacitormanufactured can be improved in the leakage current (LC)characteristics. In practice, the LC value is measured in an aqueousphosphoric acid solution after manufacturing a sintered body from theprimary powder and forming a dielectric material on the surface of thesintered body as described later. At this time, in order to reduce theLC value, the nitrided amount is preferably from 300 to 7,000 ppm bymass. The “nitrided amount” as used herein excludes the nitrogenadsorbed to niobium powder.

[0042] The nitridation of the niobium powder can be performed by liquidnitridation, ion nitridation, gas nitridation or a combination thereof.Among these, gas nitridation is preferred because the apparatus thereforis simple and the operation is easy.

[0043] The gas nitridation can be performed by allowing the niobiumpowder to stand in a nitrogen gas atmosphere. With a temperature of2,000° C. in the nitridation atmosphere and a standing time of severalhours or less, a niobium powder having an objective nitrided amount canbe obtained. The treatment time can be shortened by performing thetreatment at a high temperature. The amount of niobium powder nitridedcan be controlled by the conditions confirmed in a preliminary test orthe like on the nitridation temperature and nitridation time of thematerial to be nitrided.

[0044] The nitridation can be performed after the production of theprimary powder or after the production of a primary agglomerated powderor a secondary agglomerated powder, which are described later. Thenitridation may be performed only once after the production of eachpowder or may be performed two or more times every each production ofthe powder. For example, after a primary agglomerated powder is producedand partially nitrided and a secondary agglomerated powder is obtainedtherefrom, the secondary agglomerated powder may be re-nitrided(re-nitridation). At the time of producing the agglomerated powder, thepowder particle is deformed and the inside free of nitridation comes outto the surface in some cases but by performing the re-nitridation, thesurface of the agglomerated powder can be advantageously nitridedwithout fail.

[0045] [Primary Agglomerated Powder]

[0046] In the present invention, a niobium primary agglomerated powderresulting from agglomeration of several to tens of primary powderparticles is used as a starting material of the sintered body forcapacitors.

[0047] The primary agglomerated powder can be prepared by allowing theabove-described primary powder to stand in an atmosphere at anappropriate temperature, by cracking the powder after the standing or byfurther classifying the powder after the cracking. The primaryagglomerated powder can be produced to have any average particle size,however, a primary agglomerated powder having an average particle sizeof 0.03 to 20 μm is usually used. In the case where the primary powderis obtained by the above-described jet mill method, the primary powderneeds not be taken out from the jet mill vessel to the outside and theprimary agglomerated powder can be produced in the jet mill vessel oranother vessel connected with the jet mill, so that excess oxidation canbe advantageously prevented.

[0048] The niobium primary agglomerated powder for capacitors having anaverage circular degree of 0.8 or more, an average primary particle sizeof 0.01 to 0.5 μm and an average particle size of 0.03 to 20 μm can alsobe produced directly without using the above-described primary particle.Examples of the direct production method include hydrogen reduction ofniobium halide. A so-called continuous method of halogenating niobiummetal and further continuously hydrogen-reducing the halogenated niobiummetal may also be used.

[0049] One example of the latter continuous method is described below. Aniobium metal having an appropriate particle size of, for example, 0.1to 5 mm is filled in the first reaction tower, a halogenation gas or ahalogenation gas diluted with an inert gas such as argon or nitrogen ispassed therethrough, and the reaction is performed at a temperature of200 to 800° C., preferably from 300 to 500° C. to obtain niobium halidein the gas state. Without isolating the obtained niobium halide ordepending on the case, after adding an inert gas to dilute the obtainedniobium halide, the niobium halide is introduced into the secondreaction tower and mixed with a hydrogen gas and the reduction isperformed at a temperature of from 800 to 2,000° C., preferably from1,000 to 1,800° C., to produce a niobium metal fine powder. The gascontaining the produced niobium metal fine powder is introduced into acollection tank for niobium primary agglomerated powder and cooled. Theniobium powder may also be collected and separated from the gas using anappropriate filter or may be collected by a scrubber. At this time, ifcooling in the process of reaching the collection tank and in thecollection tank is rapidly performed, a niobium primary agglomeratedpowder is not generated and a primary powder disadvantageously results.The reduction of niobium halide with a hydrogen gas may be performedafter separating an unreacted halogen gas or an inert gas. Or, theniobium powder collected and separated from the gas may be againcontacted with a hydrogen gas to repeat the reduction reaction. Also,the niobium halide may be collected as a solid, evaporated under heatingin a separate system and reduced with a hydrogen gas. Examples of thehalogenating agent used include fluorine, chlorine, bromine and hydrogenhalogenide and these halogenating agents may be used individually or incombination of two or more thereof. The amount of the hydrogen gas usedat the reduction is preferably from 2 to 500 times in a molar ratio tothe niobium halide.

[0050] The specific surface area of the niobium primary agglomeratedpowder obtained by the above-described two methods can be freelychanged, however, a niobium primary agglomerated powder having aspecific surface area of 4 to 30 m²/g is usually used.

[0051] [Secondary Agglomerated Powder]

[0052] When the primary agglomerated powder is further granulated toform a secondary agglomerated powder having an appropriate size and acapacitor is manufactured using a sintered body of the secondaryagglomerated powder, the impregnation of cathode agent is facilitated,the capacitance can be more increased and the voltage resistancecharacteristics are improved.

[0053] In the case of manufacturing a capacitor using the secondaryagglomerated powder, excellent voltage resistance characteristics areprovided even if the average circular degree of the primary powder isless than 0.8.

[0054] The secondary agglomerated powder can be prepared by granulatingthe primary agglomerated powder according to a conventionally knownmethod. Examples of the method include a method where powder particlesare left standing at a high temperature of 500 to 2,000° C. in a vacuumand then wet or dry cracked, a method where powder particles are mixedwith an appropriate binder such as acrylic resin or polyvinyl alcoholand then cracked, and a method where powder particles are mixed with anappropriate compound such as acrylic resin, camphor, phosphoric acid orboric acid, left standing at a high temperature in a vacuum and then wetor dry cracked.

[0055] The particle size of the secondary agglomerated powder can befreely controlled by the degree of granulation and cracking, however, asecondary agglomerated powder having an average particle size of 10 to150 μm is usually used. The secondary agglomerated powder may beclassified after the granulation and cracking. After the granulation,the secondary agglomerated powder may also be mixed with an appropriateamount of powder particles before the granulation or with an appropriateamount of secondary agglomerated powders having a plurality of averageparticle sizes. The specific surface area of the thus-produced secondaryagglomerated powder can be freely adjusted, however, a secondaryagglomerated powder having a specific surface area of 3 to 20 m²/g isusually used.

[0056] [Sintered Body]

[0057] The niobium sintered body of the present invention is produced bysintering the above-described niobium primary agglomerated powder orsecondary agglomerated powder. The production method of the sinteredbody is not particularly limited but, for example, the niobium primaryagglomerated powder or secondary agglomerated powder is press-moldedinto a predetermined shape and then heated at 500 to 2,000° C. forseveral minutes to several hours under a reduced pressure of 1.33×10² to1.33×10⁻⁵ Pa (1 to 10⁻⁷ Torr) or in an inert gas such as argon, therebyproducing a sintered body.

[0058] A lead wire comprising a valve-acting metal such as niobium ortantalum may be prepared to have an appropriate shape and an appropriatelength and integrally molded at the above-described press-molding ofniobium powder such that a part of the lead wire is inserted into theinside of the molded article, whereby the lead wire can be designed towork out to a leading line of the sintered body.

[0059] The specific surface area of the thus-produced niobium sinteredbody of the present invention can be freely adjusted, however, a niobiumsintered body having a specific surface area of from 1 to 10 m²/g isusually used.

[0060] [Capacitor]

[0061] The capacitor of the present invention comprises theabove-described sintered body as one part electrode, a dielectricmaterial formed on the surface of the sintered body, and another partelectrode provided on the dielectric material.

[0062] Examples of the dielectric material for the capacitor include adielectric material comprising tantalum oxide, niobium oxide, polymersubstance or ceramic compound, with a dielectric material comprisingniobium oxide being preferred. The dielectric material comprisingniobium oxide can be obtained by chemically forming the niobium sinteredbody as one part electrode in an electrolytic solution. For chemicallyforming the niobium electrode in an electrolytic solution, an aqueousprotonic acid solution is generally used, such as aqueous 0.1% by massphosphoric acid solution or aqueous sulfuric acid solution. In the caseof obtaining a dielectric material comprising niobium oxide bychemically forming the niobium electrode in an electrolytic solution,the capacitor of the present invention is an electrolytic capacitor andthe niobium side serves as an anode.

[0063] In the capacitor of the present invention, the another partelectrode is not particularly limited and for example, at least onecompound selected from electrolytic solutions, organic semiconductorsand inorganic semiconductors known in the art of aluminum electrolyticcapacitor, can be used.

[0064] Specific examples of the electrolytic solution include adimethylformamide-ethylene glycol mixed solution having dissolvedtherein 5% by mass of isobutyl-tripropylammonium borotetrafluorideelectrolyte, and a propylene carbonate-ethylene glycol mixed solutionhaving dissolved therein 7% by mass of tetraethylammoniumborotetrafluoride.

[0065] Specific examples of the organic semiconductor include an organicsemiconductor comprising a benzenepyrroline tetramer and chloranile, anorganic semiconductor mainly comprising tetrathiotetracene, an organicsemiconductor mainly comprising tetracyanoquinodimethane, and an organicsemiconductor mainly comprising an electrically conducting polymerobtained by doping a dopant into a polymer containing a repeating unitrepresented by formula (1) or (2):

[0066] (wherein R¹ to R⁴, which may be the same or different, eachrepresents a hydrogen atom, an alkyl group having from 1 to 6 carbonatoms or an alkoxy group having from 1 to 6 carbon atoms, X representsan oxygen atom, a sulfur atom or a nitrogen atom, R⁵ is present onlywhen X is a nitrogen atom and represents a hydrogen atom or an alkylgroup having from 1 to 6 carbon atoms, and each of the pair R¹ and R²,and R³ and R⁴ may combine with each other to form a ring).

[0067] Examples of the polymer containing a repeating unit representedby formula (1) or (2) include polyaniline, polyoxyphenylene,polyphenylene sulfide, polythiophene, polyfuran, polypyrrole,polymethylpyrrole, and derivatives of these polymers.

[0068] Examples of the dopant which can be used includesulfoquinone-base dopants, anthracene monosulfonic acid-base dopants andother various anionic dopants. Also, an electron acceptor dopant such asNO⁺or NO₂ ⁺salt may be used.

[0069] Specific examples of the inorganic semiconductor includeinorganic semiconductors mainly comprising lead dioxide or manganesedioxide, and inorganic semiconductors comprising triiron tetraoxide.

[0070] These semiconductors may be used individually or in combinationof two or more thereof.

[0071] When the organic or inorganic semiconductor used has anelectrical conductivity of 10⁻² to 10³ S·cm⁻¹, the manufacturedcapacitor can have a smaller impedance value and can be more increasedin the capacitance at a high frequency.

[0072] The electrical conducting layer can be formed, for example, bythe solidification of an electrically conducting paste, the plating, themetallization or the formation of a heat-resistant electricallyconducting resin film. Preferred examples of the electrically conductingpaste include silver paste, copper paste, aluminum paste, carbon pasteand nickel paste, and these may be used individually or in combinationof two or more thereof. In the case of using two or more kinds ofpastes, the pastes may be mixed or may be superposed one on another asseparate layers. The electrically conducting paste applied is thensolidified by allowing it to stand in an air or under heating. Examplesof the plating include nickel plating, copper plating, silver platingand aluminum plating. Examples of the metal to be vapor-depositedinclude aluminum, nickel, copper and silver.

[0073] In practice, for example, aluminum paste and silver paste arestacked in this order on the another part electrode and these are moldedwith a material such as epoxy resin, thereby fabricating a capacitor.This capacitor may have a niobium or tantalum lead which is sintered andmolded integrally with the niobium sintered body or welded afterward.

[0074] The thus-fabricated capacitor of the present invention isjacketed using, for example, resin mold, resin case, metallic jacketcase, resin dipping or laminate film, and then used as a capacitorproduct for various uses.

[0075] In the case where the another part electrode is liquid, thecapacitor fabricated from the above-described two electrodes and adielectric material is housed, for example, in a can electricallyconnected to the another part electrode to complete a capacitor. In thiscase, the electrode side of the niobium sintered body is guided outsidethrough the above-described niobium or tantalum lead and at the sametime, insulated from the can using an insulating rubber or the like.

BEST MODE FOR CARRYING OUT THE INVENTION

[0076] The present invention is described in greater detail below byreferring to the Examples and Comparative Examples.

[0077] The methods for measuring and evaluating the physical propertiesin each Example are described below.

[0078] (1) Nitrogen Content of Niobium Powder

[0079] This was determined using a nitrogen and oxygen analyzermanufactured by LEKO.

[0080] (2) Average Particle Size of Agglomerated Powder

[0081] This was measured using “Microtrack” (manufactured byMicrotrack).

[0082] (3) Specific Surface Area of Agglomerated Powder

[0083] This was measured according to BET method.

[0084] (4) Voltage Resistance Value of Capacitor

[0085] This was designated as a voltage value when a voltage was appliedto 30 units of capacitors in each Example while elevating in sequence by1 V and the number of short-circuited capacitors exceeded 5.

EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLES 1 AND 2

[0086] A niobium powder (average particle size: 0.9 μm) obtained byintroducing a hydrogen gas into a niobium ingot and wet-cracking theingot was pulverized in a jet mill in a nitrogen atmosphere withoutpassing through dehydrogenation. The pulverized niobium powder was nottaken out but left standing at 400° C. under reduced pressure, therebyperforming the dehydrogenation. The dehydrogenated niobium powder wasfurther left standing at 850° C. and then cracked to produce a niobiumprimary agglomerated powder. The values of the thus-produced primaryagglomerated powder are shown in Table 1. The average circular degreewas adjusted by varying the residence time in the jet mill.

[0087] Subsequently, a nitrogen gas was passed therethrough at 300° C.for 20 minutes to obtain a partially nitrided primary agglomeratedpowder having a nitrided amount of about 1,600 ppm.

[0088] The obtained primary agglomerated powder was molded to a size of1.8×3.5×4.5 mm (a niobium wire having a diameter of 0.3 mm wasintegrally molded to work out to a lead) and then sintered at 1,250° C.in a vacuum of 6.7×10⁻³ Pa (5×10⁻⁵ Torr) to obtain a sintered body. Thissintered body was electrochemically formed in an aqueous 0.1% by massphosphoric acid solution at a temperature of 80° C. and 20 V to form adielectric layer comprising niobium oxide. Thereafter, polypyrrole(using ammonium persulfate as an oxidant and sodium anthraquinonesulfateas a dopant, the reaction between pyrrole and oxidant was repeated toeffect polymerization in the presence of the dopant) as another partelectrode material was filled in pores inside the sintered body.Furthermore, carbon paste and silver paste were stacked in this order onthe another part electrode and after mounting on a lead frame, the wholewas molded with an epoxy resin to manufacture a capacitor. The specificsurface area of the sintered body in each Example, and the capacitanceand the voltage resistance of the manufactured capacitor are shown inTable 2. TABLE 1 Average Average Particle Size Average Particle Size ofPrimary Specific Circular of Primary Agglomerated Surface Degree Powder,μm Powder, μm Area, m²/g Example 1 0.89 0.3 14 9 Example 2 0.84 0.3 1010 Example 3 0.80 0.4 9 7 Comparative Example 1 0.77 0.4 9 7 ComparativeExample 2 0.75 0.4 9 7

[0089] TABLE 2 Specific Surface Area Volume of Sintered Body CapacitanceResistance (m²/g) (μF) (V) Example 1 3.0 1100 13 Example 2 3.2 1270 12Example 3 2.6 940 13 Comparative Example 1 2.6 640 8 Comparative Example2 2.6 570 8

[0090] As is apparent from Tables 1 and 2, when a primary agglomeratedpowder comprising primary powder having an average circular degree of0.8 or more is used, the capacitor is improved in the capacitance andthe voltage resistance characteristics as compared with the case using apowder having an average circular degree of less than 0.8 (Examples 1 to3 and Comparative Examples 1 and 2). In particular, on comparison of theresults between Example 3 and Comparative Example 1 where only theaverage circular degree value of the primary powder is different andother conditions are the same, it is seen that excellent characteristicscan be obtained by using a primary powder having a high average circulardegree.

EXAMPLE 4

[0091] A high-purity niobium metal having a particle size of about 5 mmwas filled in the first reaction tower and heated at 400° C. Thereafter,a dry chlorine gas diluted with argon gas to 50% by volume was passedthrough that metal niobium layer and reacted. The resulting reaction gaswas introduced into the second reaction tower designed such that thereaction gas blows out through a capillary, and from the peripherythereof, hydrogen gas was fed in a molar amount of 20 times the reactiongas (NbCl_(x)) and reacted at 1,200° C. The reaction product wasintroduced into the recovery tower for niobium primary agglomeratedpowder and after separating gases such as hydrogen chloride, broughtinto contact with hydrogen, followed by water washing and drying.Subsequently, a very slight amount of air diluted with nitrogen wasrepeatedly contacted with the powder, as a result, a primaryagglomerated powder having no excess oxide film on the surface thereofand having an average particle size of 0.3 μm was obtained. This primaryagglomerated powder was an agglomerate of several to ten primary powderparticles having an average circular degree of 0.94 and the specificsurface area thereof was about 21 m²/g.

[0092] The obtained primary agglomerated powder was left standing at780° C. under reduced pressure of 6.7×10⁻⁴ Pa (5×10⁻⁶ Torr) and thencracked to obtain a secondary agglomerated powder having an averageparticle size of 85 μm and a specific surface area of 13 m²/g.Subsequently, this secondary agglomerated powder was left standing at300° C. for two hours in a nitrogen gas stream to obtain a partiallynitrided secondary agglomerated powder having a nitrided amount of 1,200ppm.

[0093] Using this secondary agglomerated powder, a molded article andthen a sintered body were obtained in the same manner as in Example 1except for changing the sintering temperature to 1,150° C. The specificsurface area of the sintered body was 6.8 m²/g.

[0094] In the same manner as in Example 1, a niobium oxide dielectriclayer was formed on the surface of the sintered body. Then, using amixed solution of an aqueous 30% lead acetate solution and an aqueous35% ammonium persulfate solution, an inorganic semiconductor layercomprising lead dioxide and lead sulfate (lead dioxide: 95% by mass) wasformed as another part electrode layer on the dielectric layer.Thereafter, in the same manner as in Example 1, a carbon layer and asilver paste layer were stacked in this order on the another partelectrode layer and after mounting on a lead frame, the whole was moldedwith an epoxy resin to manufacture a capacitor.

EXAMPLES 5 AND 6

[0095] The primary agglomerated powders produced in the same manner asin Example 3 and Comparative Example 1 each was left standing at 950° C.under reduced pressure of 6.7×10⁻³ Pa (5×10⁻⁵ Torr) to obtain asecondary agglomerated powder (the secondary agglomerated powder fromExample 3 was Example 5 and the secondary agglomerated powder fromComparative Example 1 was Example 6). Each secondary agglomerated powderwas classified to obtain secondary agglomerated powders having anaverage particle size of 120 μm and a specific surface area of 6.2 m²/g.Subsequently, molded articles were manufactured in the same manner as inExample 1 and then sintered to obtain sintered bodies. These sinteredbodies both had a specific surface area of 2.2 m²/g. Thereafter, in thesame manner as in Example 1, a niobium oxide dielectric layer, anotherpart electrode layer comprising polypyrrole, a carbon layer and a silverpaste layer were formed in this order and the whole was molded with anepoxy resin to manufacture capacitors.

[0096] The capacitance and the voltage resistance value of eachcapacitor manufactured in Examples 4 to 6 are shown in Table 3. TABLE 3Volume (μF) Voltage Resistance (V) Example 4 2160 13 Example 5 970 12Example 6 1000 12

[0097] It is seen from Tables 1, 2 and 3 that the capacitors using asecondary agglomerated powder (Examples 5 and 6) have a largercapacitance and a higher voltage resistance than the capacitors using aprimary agglomerated powder (Example 3 and Comparative Example 1), thateven in the case of using a primary powder having an average circulardegree of less than 0.8, an excellent effect can be obtained by formingit into a secondary agglomerated powder, and that when a secondaryagglomerated powder granulated from a primary powder having a highcircular degree (circular degree: 0.94) through a primary agglomeratedpowder by a continuous method is used, the obtained capacitor can have alarge capacitance and good voltage resistance characteristics (Example4).

INDUSTRIAL APPLICABILITY OF THE INVENTION

[0098] According to the present invention, the starting material niobiumpowder of a sintered body for niobium capacitors is a niobium primaryparticle having an average particle size of 0.01 to 0.5 μm and beingspecified that the circular degree of primary particles is 0.8 or more,a primary agglomerated powder obtained by granulating the niobiumprimary powder, or a secondary agglomerated powder obtained by furthergranulating the primary agglomerated powder. By sintering this startingmaterial niobium powder and using the obtained sintered body, acapacitor having a large capacitance per unit volume and good voltageresistance characteristics can be manufactured.

[0099] In the case where a fine metal powder is used as it is and formedinto a sintered body, the pore size of the sintered body decreases orclosed pores increases at the stage of sintering and therefore, thecathode agent cannot be satisfactorily impregnated. However, accordingto the present invention, a fine niobium primary powder for capacitorshaving an average particle size of 0.01 to 0.5 μm is granulated to anappropriate size to produce a primary agglomerated powder, or theproduced primary agglomerated powder is again granulated to produce asecondary agglomerated powder, whereby, it is considered, pores havingdifferent sizes are disposed inside the sintered body, as a result, theimpregnation of the cathode agent can be facilitated, the impregnationrate can be increased and the capacitance per unit volume can beincreased.

[0100] Furthermore, although the voltage resistance characteristics of acapacitor is considered to depend on the mechanical concentrated stressand the electric current stress generated at the edge part of a rawmaterial powder particle and also on the above-described impregnationrate of the cathode agent, the shape of the raw material niobium powderparticle of the present invention is approximated to a sphere and theedge part of the powder particle is reduced, whereby, it is considered,the voltage resistance characteristics of the capacitor are improved.

1. A niobium primary powder for capacitors, having an average circulardegree of 0.8 or more and an average particle size of 0.01 to 0.5 μm,the circular degree being defined by the following formula: Circulardegree=4 πA/L² (wherein A is an area of a solid projected on a flatsurface and L is the outer circumferential length of the projectionview).
 2. The niobium primary powder for capacitors as claimed in claim1, wherein at least a part of niobium is nitrided.
 3. A niobium primaryagglomerated powder for capacitors, having an average primary particlesize of 0.01 to 0.5 μm and an average particle size of 0.03 to 20 μm. 4.The niobium primary agglomerated powder for capacitors as claimed inclaim 3, wherein the primary particles have an average circular degreeof 0.8 or more, the circular degree being defined by the followingformula: Circular degree=4 πA/L² (wherein the symbols have the samemeanings as in claim 1)
 5. The niobium primary agglomerated powder forcapacitors as claimed in claim 3 or 4, wherein at least a part ofniobium is nitrided.
 6. The niobium primary agglomerated powder forcapacitors as claimed in claim 3 or 4, wherein the specific surface areais from 4 to 30 m²/g.
 7. A niobium secondary agglomerated powder forcapacitors, obtained by granulating the niobium primary agglomeratedpowder claimed in any one of claims 3 to
 6. 8. The niobium secondaryagglomerated powder for capacitors as claimed in claim 7, wherein thespecific surface area is from 3 to 20 m²/g.
 9. The niobium secondaryagglomerated powder for capacitors as claimed in claim 7 or 8, whereinthe average particle size is from 50 to 150 μm.
 10. The niobiumsecondary agglomerated powder for capacitors as claimed in any one ofclaims 7 to 9, wherein at least a part of niobium is nitrided.
 11. Asintered body using the niobium primary agglomerated powder forcapacitors claimed in any one of claims 3 to
 6. 12. A sintered bodyusing the niobium secondary agglomerated powder for capacitors claimedin any one of claims 7 to
 10. 13. The sintered body as claimed in claim11 or 12, wherein the specific surface area is from 1 to 10 m²/g.
 14. Acapacitor comprising the sintered body as claimed in claim 11 or 12 asone part electrode, a dielectric material formed on the surface of thesintered body, and another part electrode provided on the dielectricmaterial.
 15. The capacitor as claimed in claim 14, wherein thedielectric material mainly comprises niobium oxide.
 16. The capacitor asclaimed in claim 15, wherein the niobium oxide is formed by electrolyticoxidation.
 17. The capacitor as claimed in any one of claims 14 to 16,wherein the another part electrode is at least one material selectedfrom the group consisting of an electrolytic solution, an organicsemiconductor and an inorganic semiconductor.
 18. The capacitor asclaimed in claim 17, wherein the another part electrode is composed ofan organic semiconductor and the organic semiconductor is at least oneorganic semiconductor selected from the group consisting of an organicsemiconductor comprising a benzopyrroline tetramer and chloranile, anorganic semiconductor mainly comprising tetrathiotetracene, an organicsemiconductor mainly comprising tetracyanoquinodimethane, and an organicsemiconductor mainly comprising an electrically conducting polymerobtained by doping a dopant into a polymer containing two or morerepeating units represented by the following formula (1) or (2):

wherein R¹ to R⁴, which may be the same or different, each represents ahydrogen atom, an alkyl group having from 1 to 6 carbon atoms or analkoxy group having from 1 to 6 carbon atoms, X represents an oxygenatom, a sulfur atom or a nitrogen atom, R⁵ is present only when X is anitrogen atom and represents a hydrogen atom or an alkyl group havingfrom 1 to 6 carbon atoms, and each of the pairs R¹ and R², and R³ and R⁴may combine with each other to form a ring.
 19. The capacitor as claimedin claim 18, wherein the organic semiconductor is at least one memberselected from polypyrrole, polythiophene polyaniline and substitutionderivatives thereof.